This invention relates to digital logic circuits implemented with flux flow transistors made from high temperature superconducting materials.
There are at least two other superconducting technologies that can be used for logic circuits. Modified Variable Threshold Logic (MVTL) and Single Flux Quantum (SFQ) are both based on small Josephson junctions. MVTL uses hysteretic Josephson junctions in a latching mode. While on the same order of speed as the flux flow logic circuits disclosed in this application, they have difficulty in interfacing directly to conventional electronics. The MVTL technology is explained in the reference by S. Hasuo, S. Kotani, A. Inoue and N. Fujimaki, "High Speed Josephson Processor Technology" IEEE Trans. on Mag. MAG-27, 2602 (1991). SFQ uses non-hysteretic junctions and circuit topologies that operate on individual vortices. This family is faster than flux flow logic, but certain combinational functions are more difficult to implement. Additionally, they suffer from noise immunity and cross-talk problems and have difficulty in interfacing to conventional electronics. The SFQ logic is explained in the reference by K. K. Likharev and V. K. Semenov, "RSFQ Logic/Memory Family: A New Josephson-Junction Technology for Sub-Terahertz-Clock-Frequency Digital Systems" IEEE Trans. on Appl. Supercond. 1, 3 (1991). Memories have also been difficult to implement in these two technologies because of standby power dissipation and coupling problems.
The flux flow logic described herein has important advantages. The flux flow flip flop and inverter have very low power dissipation in all static states, and the devices can couple trivially to each other and to conventional circuitry. Compared to conventional logic circuits, generally FET-based, the flux flow circuits have major advantages in power dissipation (by orders of magnitude) and speed. For the same linewidth lithography, it is estimated that flux flow logic is more than an order of magnitude faster than GaAs-based FET logic. Because the flux flow signal levels are reasonable, they can couple effectively to conventional circuitry. The basic operation of the SFFT is set forth in U.S. Pat. No. 5,019,721 for "Active Superconducting Devices Formed of Thin Films" issued on May 28, 1991 to J. S. Martens et al. This patent is incorporated by reference in its entirety.